General Anesthetics

General anesthesia act on the brain and nervous system. They affect the entire body leaving the patient in a deep sleep.

Most hip surgery is now carried out using a regional anesthetic in conjunction with a light general. Why? Because general anesthetics are expensive and have multiple side effects.

Administration

Administration of general anesthesia has two stages: -

Induction - putting you to sleep

Maintenance - keeping you under

Induction of anesthesia

In contemporary practice intravenous anesthetics are usually used for induction. Propofol is probably the most widely used. It has replaced the long standing barbiturate agents such as thiopentone because recovery from propofol is faster and has fewer post anesthesia side effects (see below).

Propofol is not an analgesic, so is administered in combination with an opioid such as fentanyl. Generally speaking a patient won't feel pain whilst under a general anesthetic unless the effectiveness of the anesthetia weakens during the procedure. If that happens the opioid is already in place. Fentanyl, and other opioids, also provide excellent post-operative pain relief.

Inhalational induction (fairly rare) is carried out using an agent called sevoflurane which is a non-pungent gas.

Maintenance of anesthesia

Following induction, inhalational anesthetics or intravenous anesthetic agents are administered continuously to maintain an adequate depth of anesthesia - using the same anesthetic drugs as described above.

All patients are given an inhalation mix of nitrous oxide and oxygen (even if the maintenance anesthetic is given intravenously).

Nitrous oxide: in addition to its analgesic effect it also has a weak anesthetic effect. This means that the dosage of the main general anesthetic can be reduced.

Oxygen: makes up about 30 percent of the total inspired mixture. The main role, of course, being to ensure the patient can breathe but importantly it also ensures that the concentration of nitrous oxide is kept within safe limits.

Risks and Warnings

As with any anesthesia there are risks involved (hence all the care that is taken during your preoperative assessment to ensure you are in a fit state to undergo the operation). These risks are increased in patients who have heart disease or a chronic lung condition.

General anesthesia slows both your heart rate and breathing rate. The anesthetist will constantly monitor your heart rate, blood pressure, breathing, and body temperature so that action can be taken to prevent the heart rate dropping too low (bradycardia)

General anesthesia causes your blood vessels to dilate; this can result in a heavier loss of blood during surgery.

The tube inserted down your throat may give you a sore throat and hoarse voice for a few days.

Shivering is a frequent occurrence in the post-operative period. Apart from causing discomfort and exacerbating post-operative pain, shivering has been shown to increase oxygen consumption, catecholamine release (which stimulates all the systems of body), cardiac output, heart rate, blood pressure and intra-ocular pressure. This can be prevented by adjusting the ambient temperature in the operating theater or treated by the use of conventional or forced warm air blankets and warmed intravenous fluids

Post-operative affects such as emergence delusion, headaches, nausea and vomiting and significant drowsiness are also associated with general anesthetics

Advantages

The patient is completely unconscious so has no awareness of the procedure.